Hydrocarbon exploration is the search for oil and gas accumulations in the subsurface of the Earth. Oil and gas exploration is typically high risk, with most oil wells that are drilled failing to find hydrocarbons or having too little production to be economic. Since oil and gas wells are expensive, ranging in cost from $100,000 for shallow onshore wells to $100 million for deep offshore wells, there is great incentive to improve the probability of exploration success.
Thus, various geophysical tools are used by geophysicists and geologists to assist in hydrocarbon exploration, including gravity, magnetics, and seismic methods. These geophysical tools typically look for structural changes in the subsurface where oil and gas may be trapped, for example, at faults or anticlinal structures.
Other types of geophysical exploration tools are electrical resistivity and electromagnetic surveys. These exploration tools rely on the principle that brine is an electrical conductor while oil and gas are electrical insulators. Thus if oil and gas are present in the pore space of a reservoir rock, the electrical resistivity is higher than if brine only were present.
In a petroleum prospect, a source rock containing solid organic matter (kerogen) is buried to deeper subsurface depths over millions of years as more sediment is deposited on top. With increasing time and temperature, the kerogen gradually matures to a liquid hydrocarbon called bitumen, which is rich in polar compounds and thus adheres to the kerogen surface. With still further maturation, the bitumen matures to a mobile oil which displaces some brine from the source rock. The electrical resistivity of the source rock increases as brine is displaced from the pores of the source rock. Some of the oil typically migrates from the source rock to a reservoir rock under the combination of influences of buoyancy (the oil is lighter than the brine), compaction of the pore space of the source rock as it is buried deeper, and thermal expansion of the fluids.
Van Krevelen classified the types of kerogen into three broad classes based on their hydrogen and oxygen indices. Type I kerogens have a high hydrogen index (HI) and low oxygen index (OI); they are derived from algal matter deposited in lacustrine environments. Type II kerogens are also high HI but higher OI and are derived from marine organisms in deep marine environments; and Type III have low HI and high OI and are derived from woody matter in shallow swamps and lagoons. Type II-s kerogens are a sub class of Type II kerogen having in addition high sulfur content. The weight percent sulfur in Type II-s kerogen can range up to about 10 wt %.
In most cases, when hydrocarbon deposits are found at shallow depths, it is because the oil and gas has migrated into a reservoir rock after being generated and expelled from a much deeper source rock. The maturation history of most kerogens (Types I, II and III) requires the source rock to be buried very deeply in order to begin generation of bitumen and oil, typically 3 kilometers or more. However, because the sulfur-sulfur and sulfur-carbon bonds in the Type II-s kerogen are more thermally unstable, the generation of bitumen and oil may begin at surprisingly lower temperatures or depths in a Type II-s kerogen.